The invention relates to a hydraulic control system including pump means comprising a plurality of hydraulic fixed displacement pumps, and valve means incorporating hydraulic control circuitry for unloading a pump in response to load pressure or rate of flow of operating fluid, thereby leading to quieter operation of hydraulic systems utilizing the pump.
The invention is especially adapted for use in control systems for operating a vehicle mounted hydraulic implement such as the refuse packer on a truck. One important aspect of the invention relates to a vehicle having a motor for powering the vehicle which motor also drives the pumps providing power for the hydraulic system.
The use of the invention leads to substantial reductions in the noise level attendant with prior art hydraulic systems when operating at high pressure or high engine speed, and can be used in a manner which facilitates quieter operation of refuse packing trucks and other equipment without impairment of the normal operating routine of those who work on the equipment.
According to the invention, a system is provided which incorporates two fixed displacement pumps which deliver hydraulic fluid for operating auxiliary mechanisms such as a hydraulic ram or other hydraulically operated work piece sometimes herein called the load. In a typical system a hydraulic pump is driven for example, via a power takeoff from the engine for propelling a truck or piece of earth moving equipment.
In a typical hydraulically operated packer, a single relatively large pump is used and for adequate flow of hydraulic fluid, the pump must be driven at a relatively high speed. When the vehicle on which the hydraulic system is mounted is at a standstill, the operator must speed up the vehicle engine in order to provide the necessary flow of fluid. The high speed of the engine at a time of relatively low horsepower requirement contributes a large percentage of the noise which is so objectionable to those in the vicinity. Coupled with this is the fact that the pump is also operating at high speed and pump noise goes up rapidly with speed.
With the multiple pump system provided by the invention, adequate flow of hydraulic fluid is provided with the engine running just above low idle speed and very little noise is generated from either the vehicle engine or the pump. At such low engine speeds the engine develops relatively low torque, but where load pressure is low, as for instance through about 90% of a packing cycle, there is adequate torque available to drive the multiple pump. During the latter portion of the cycle when load pressure is high and engine torque is inadequate to drive both pumps, the invention provides for unloading one pump in response to pressure. This immediately reduces the volume of operating fluid being pumped and accordingly reduces the torque required to turn the remaining pump section so that the high pressure required during the remaining portion of the packing cycle can be achieved. Since only a small portion of the packing cycle remains, the time required for completion of the packing cycle is increased very little.
The invention further provides a capability for shifting out one pump section in response to increase in engine speed above a predetermined level. An important advantage of this capability is that the hydraulic system can be operated while the vehicle is moving from one point to another without overtaxing the horsepower of the engine. In a refuse packer for example, when the truck is moving from one pickup station to another, a major portion of the engine horsepower is needed to propel the vehicle. With the invention an increase in engine speed above a predetermined level causes one section of the multiple pump to unload so that adequate horsepower is available for both functions. In addition, the unloading of one pump section in response to flow decreases the noise of the hydraulic system. As indicated, the use of the invention provides for relatively rapid compaction of the refuse throughout a substantial portion of the compaction stroke. Although the time interval required to complete compaction of trash may be lengthened, complete compaction can be effected at times when it does not interfere with the work routine of the individuals working on the truck, as for example when the truck is driven between pickup points or is being returned to the refuse disposal site.